Volcanic Ash Beds in the Waikato District W
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EARTH SCIENCE JOURNAL, Vol. 1, No. 1, 1967 VOLCANIC ASH BEDS IN THE WAIKATO DISTRICT W. A. Pullar ' New Zealand Soil Bureau INTRODUCTION This report lies somewhere between the "pathfinder" variety and the completed account for the reason that the results of detailed mapping and identification are still being prepared for pUblication. For the younger beds less than 36,000 years we now know both the source and the distribution, but for the older ashes commonly referred to as the Hamilton ash, sources are unknown and a knowledge of distribution restricted to the Waikato district. The principal source is the Okataina volcanic centre with Taupo as a subsidiary (Healy, 19'64; Thompson, 1964 :44), and on this informa tion, current mapping into the Waikato district proceeds from the ea.st. Under the circumstances of partly completed work it seems prudent to discuss relevant ash beds already known (Vucetich and Pullar, 1963 :65-6; 19-64 :45-6) to introduce briefly current work by the same authors and by W. T. Ward, and then to relate all of this to previous work portrayed in a soil-forming ash shower map by Taylor (1953). ASH STRATIGRAPHY The beds are described in order of increasing age and with reference to the correlation line, Mamaku plateau-Parewera (Fig. 1). F'or some, descriptive names have had to be borrowed from Vucetich and Pullar (1963 :65-6) until formal geographic names are allotted later. Tarawera Ash (erupted 1886 from Mt. Tarawera); Eastern Waikato District may have received a dusting but no sighting documented. Kaharoa Ash (erupted from Mt. Tarawera 930 ± 70 years before 1950) : measurable deposit is about 1 in. on Mamaku plateau and about 2 in. at Waihi Beach. Fine white glassy grains have been noted in the surface peat at Hamilton and these could represent Kaharoa Ash. Taupo Pumice (Fig. 2) Largely Taupo Lapilli (Member 3 of Healy, 1965 :65); erupted 130 A.D. from Waitahanui. Pronounced lobe at Tirau where the bed is 6 in. thick and thinning towards Hamilton. The lapilli may be angular and jagged and appear to be less yellow and more greyish white than those at Te Whaiti and further east, and as well, are more clear-glassy and honey combed and so easier to crush in the hand. They are also less obviously vesicular and may have flow texture with flattened or eliptical vesicles. These attributes suggest a later phase in the eruption of the Taupo Lapilli regarded as an isolated shower (Ewart, 1963 :399). Air-fall Taupo Pumice at Hamilton is discussed in more detail by P. J. Tonkin (Appendix A). 17 Progressive air -line miles, --~-- 1 - 1 1-- o 10 17 21 32 4,? I!I!j ~. l"l ~ ,f:!! IQ ~ -0 01 """ 0 D: 1'» I) '[ , 0. ~ , I::: • o g.rn _I"} 0 .... .e(\l o'<tt"l Q.~ .... 10 ..... ::>10 o 0I1Dt"l S::::r' 01(\1 , (\I c(J)(J) ::>(\1 1::: .... 4)(\1 2~ L SI') oC() I) 0...,::: ~l"l . ~ ~ LID s:: .... L o'<t v .... 010 ~ 0 ..... 0 =0 s:: 1::: o L. -_ E;j) .... ID ... ::>ID ... 0 ..... olD- o'<t» I) ID ._ ID olD o ......... " ~[ ~Z :X:Z Q.Z ~z Q. o Zt- 1'1 m £' 8 0 Taupo --------·I~ $1)= Rotoma s ~~ --------------- '--'-- Bed 1 ~$I) <: Rotorua -~.... rn ~~ tAl ..... -a ~' S Hamilton ash. ~ $I) S ·Pinkish Brown- rr beds. '0..... ~ ~ l£!' Yellow andE ~ White Block r+ o beds, o r+- Ash Columns: ~ Grey Banded g. bed, ~ 15' O'Q. MAMAKU PLATEAU - TIRAU ~ og- PIARERE - MAUNGATAUTARI - PARAWERA g~ Hamilton ash. $I) 20' M.J.VENNARO DISTR IBUTION OF TAUPO PUMICE ~ () &q' E; (D () N ~ m q- a~ §. o..... ~ 11" ~ o 6 'g -Te' Kuiti (':Ie. - Matawai Cl) F.& 10 co Cl) 12 ~Cl) 9 p.. 8 > ....~ 9 9 c:..:J P 4 7 -~ _Taumarunui o 'C 11' (':I m0" tot NOTE: m$. Isopachs. and plotted thicknesse's in. inches a unless otherwise shown . ....co 0) Only air-fall tephra shown. 0) Nue'e ardente material '-' not shown. -Ohakune o 50 Scale in Miles Rotoma Asb (Fig. 3) Erupted from Eastern end of Lake Rotoma ca. 8,000 years ago. At the source, mineral assemblage comprises plagioclase, quartz, hornblende, hypersthene and magnetite with hypersthene increasing towards the top. On Mamaku plateau, the upper part is blackish grey and the lower is a pale grey ash with a thin discontinuous layer of pale yellow fine ash (silt) at base. At Tirau, where the ash becomes incorporated in the soil, shower bedding is unrecognisable but the material is fluffy, loose and dry with a sharp break at 11 in. This ash is not identifiable with certainty west of the Mamaku plateau, but is thought to be associated with the sandy portions of topsoils, at least, as far west as Kiwitahi School near Morrins ville where a thickness of 6 in. was recorded. Rotorua Ash (Fig. 4) Erupted from Lake Tikitapu; date unknown but thought to be about 13,000 years ago. On the Mamaku plateau it is identified by its rusty lapilli, which at the summit, change sharply to smalllapilli and coarse ash, and at Fitzgerald's Glade to coarse ash. At Tirau the bed becomes a yellow ish brown fine sandy loam with grains of rusty coarse ash, but further west, the latter seem to disappear. It is likely, however, that Rotorua Ash mantles most of the eastern Waikato district. Eventually, refugia con taining identifiable Rotoma and Rotorua ashes will be found. Pinkish Brown Beds (Vucetich and Pullar, 1963:65; 70) Three beds, the upper two of which are thought to have been erupted from near Mt. Tarawera about 20,000 years ago and the lower a little earlier from near Taupo. The lower bed is noted for chalazoidites (volcanic hailstones) particularly at Ngongotaha, Ngakuru basin, and at Oruanui near Wairakei. The beds have also been called "X" beds and "mauve" beds (op. cit.) but from extensive mapping are now known to be the same; also comprise part of the mantle of Late Quaternary brown ashes (a) (Healy et aI, 19'64). For purposes of this account they are referred to as "pinkish brown beds". On the Mamaku plateau, the upper two of pale brown ash are separated by beds of loess (Fig. 1) and the lower is a grey greasy silt with a thin basal layer of white ash. At Tirau, the upper two collapse into a single bed and this along with the lower persists well into the eastern Waika to District (Fig. 1). Yellow Block and White Block Beds (Vucetich and Pullar, 1963 :65, 66, 70) ; Late Quarternary pumice block and lapilli beds (b) and (c) (Healy et aI, 1964) : erupted possibly from two centres at Matahina and Rotoehu about 27,000 years ago. On Mamaku plateau (Fig. 1), the beds are recognised by their pale yellow rotten lapilli which on the Kaimai range and coastal hills from Te Poi to Tauranga and Katikati, change to reddish yellow small lapilli and coarse ash crushing easily to clay. At Tirau, the beds collapse to a pinkish brown or mauve hard sandy clay 4 to 19 in. thick, with marked vertical cracking and giving prismatic structure as in a paleosol; in road cuttings, the bed is marked by a "nick" sculptured by wind and rain. At Parewera, however, the bed protrudes as a firm massive silt loam with vertical cracks and reacts strongly to an allophane field test. The pumice block and lapilli beds (b) and (c) differ from Rotoma Ash and Rotoiti Breccia in that they are dacitic in composition (Ewart and Healy, 1965; 26). They contain no quartz, but have plagiocIase, hypers thene, augite and magnetite. 20 DISTRIBUTION OF ROTOMA ASH. ~ t: " I !2~ 0 , f I .'1"8~~~~ 0 ~ t:I .....m c+ :1. 0" a§ 0 ~ ~ ~ 0 SI)!3 SI) !- (I) 't:$a w c+ ~ (I) ~ (') ~ co 0 0 0 l.<l (I) SI) ~ SI) ~ ,....9 J-'I .TaumQrunui rn 0 '0 g-SI) m ~..... IsopachsNom a nd plotted I 3 m Ithicknesses in inches. (I) ~ unless otherwise shown. co~ I Cl) ......Cl) I -Ohakune ( 0 50 Scale in M i les M. I'Ij DISTRIBUTION O'F ROTORUA ASH. dq = C) ~'"' r s-'\\ ~ () I // 9~~~ ~ m ..t- ~. 0''"' ..t-s:: o· L I 11 a _'I,lb "- '-?1' .............. C l:l • a ~ 0 ..t-o ~ $U SI' m ::r' Cl) ~ leTe Ku iti .. V ~ ') .... \1:) \19"-- J J" a r ·Matawai Cl) N P. N (") P ..... Cl.) 0 0 0 c..:: Cl) \) SI' t; m -VJ $U ~ ~ Otl i ? eTaumarunui ,.....,. ~ m 0 I Id $U (") ::r' m NOTE t; IIsopachs and plotted ~. thicknesses in inches m unless otherwise shown . Cl)p. I ..... CC CJ:I CJ:I '-" • eOhakune 0 50 f Scale in Miles Grey Banded Bed (Vucetich and Pullar, 1963:66, 70); Late Quaternary volcanic ash member (d) (Healy et aI, 1964): Shower bedded ash associated with the Rotoiti Breccia eruption and erupted from a little north of Lake Rotoiti about 36,000 years ago. The mineral assemblage comprises plagioc1ase, quartz, hypersthene, hornblende, and magnetite (Ewart and Healy, 1965 :26). This bed of whitish grey ash is distinguished by its fine shower bedding which persists well into the Waikato District; the contact with the underlying brown and reddish brown Hamilton beds is always clear and sharp. Relationship of Previous Work to Current Work In this report, previous work refers more to that of Taylor and Grange and distribution to that of Taylor's map (1953). For convenience, previous work is set up and then examined and criticised. Tirau Ash Not defined in Fleming (1959).